Three-phase brushless direct-current (DC) motors have many uses, among which include both high-speed and low-speed applications. Conventional high-speed applications include spindle motors for computer hard disk drives, digital video disk (DVD) drives, compact-disk (CD) players, tape-drives for video recorders, and blowers for vacuum cleaners. A motor for high-speed applications typically operates in a range from a few thousand revolutions per minute (RPM's) to 20,000 RPM's, for example. Conventional low-speed applications include motors for farm and construction equipment, heating, ventilation, and air-conditioning (HVAC) compressors, and fuel pumps. Motors for low-speed applications typically operate in a range from less than a few hundred RPM's to a few thousand RPM's, for example. Compared to DC motors employing brushes, brushless DC motors enjoy reduced noise generation and improved reliability because no brushes need to be replaced due to wear.
A brushless DC motor includes a permanent magnet rotor and a stator having a number of windings, typically three and often referred to as phases A, B, and C. The windings are each formed in a plurality of slots in the stator. Often, the rotor may be housed within the stator, but in some applications, the stator may also be housed within the rotor. The rotor is permanently magnetized, and turns to align its own magnetic flux with the flux generated by the windings when current flows through the windings. As such, when power is supplied to the windings (as current through the windings), the rotor will be pushed or pulled in a specific direction due the magnetic flux created by the current.
Power to the motor is often provided in a pulse width modulation (PWM) mode. The PWM mode is a nonlinear mode of power supply in which the power is switched on and off at a very high frequency in comparison to the angular velocity of the rotor. In order to operate the motor, the flux existing in the stator is controlled to be as quadratic as possible with the rotor flux, thereby continually pulling the rotor forward. Therefore, to optimize the efficiency of the motor, it is advantageous to monitor the position of the rotor so that the flux in the stator may be appropriately controlled and switched from one commutation stage to the next in the commutation sequence. If the rotor movement and the flux rotation should ever get out of synchronization, the rotor may become less efficient, start to jitter, or stop turning.
When first starting up, coils of the brushless DC motor are energized in a proper sequence such that the magnetic flux generated will initiate or continue to drive the rotor in the proper direction. Therefore, the rest or low speed position of the rotor relative to the stator is determined before starting up the motor so as to initiate the proper drive signal. Conventional methods for determining the beginning position of the rotor relative to the stator may be inefficient and time-consuming approaches. One general technique for detecting the starting position of the motor at standstill or at low speed is to use external sensors, such as hall sensors or optical sensors, which can provide the rotor position information. However, this technique requires external sensors, extra circuitry to process the sensor signals, and increases the parts count and the size of the circuit board. Such cumbersome solutions are inefficient and wasteful of time and energy.